1. Field of the Invention
The present invention relates to an apparatus for cleaning a transfer roller. More particularly, the present invention relates to an apparatus and a method for cleaning a transfer roller of a wet-type image forming apparatus so as to remove a contamination of the transfer roller by transferring an image formed on an intermediate transfer medium to a printing paper in the wet-type image forming apparatus using a liquid developer.
2. Description of the Related Art
In general, image forming apparatuses may be classified into dry-type image forming apparatuses that use a powdered dry-type developer and wet-type image forming apparatuses that use a liquid developer. The dry-type and wet-type image forming apparatuses each have their own advantages and disadvantages. However, the wet-type image forming apparatus can obtain a clearer, higher resolution printed document than the dry-type image forming apparatus and thus is particularly suitable for color printing. In the case of the wet-type image forming apparatus, a developer including a mixture of powdered toner with a predetermined color and a liquid carrier for assisting movement of the toner is used during developing and transferring processes.
FIG. 1 is a schematic view of a general electrophotographic wet-type image forming apparatus.
In the general electrophotographic wet-type image forming apparatus, electrostatic latent images are formed on four photosensitive bodies 11, 12, 13, and 14, which are charged with predetermined potentials via charging units (not shown), by laser beams emitted from exposing units (not shown). The four colors are typically cyan (C), magenta (M), yellow (Y), and black (K). Developers with predetermined colors are respectively supplied to the photosensitive bodies 11, 12, 13, and 14 so as to develop the electrostatic latent images as visible images. First transfer rollers 21, 22, 23, and 24 transfer the visible images to an intermediate transfer belt (ITB) 30 50 that the visible images are overlapped on the ITB 30. A second transfer roller 36 transfers the overlapped image to a printing paper P. ITB 30 is supported by and revolves around support rollers 32, 33, first transfer rollers 21, 22. 23, and 24, and transfer backup roller 34. The printing paper P to which a color image is transferred undergoes a series of fixing processes and then is discharged outside the electrophotographic wet-type image forming apparatus.
When a final image formed on the ITB 30 is transferred to the printing paper P, the second transfer roller 36 contacts the ITB 30 and is supplied with a high voltage so as to transfer the final image to the printing paper P due to an electrostatic attraction.
A transfer characteristic of a transfer system of the electrophotographic wet-type image forming apparatus varies depending on a transfer voltage applied between the second transfer roller 36 and a transfer backup roller 34 for a transfer. When the transfer voltage is low, the electrostatic attraction generated from the second transfer roller 36 is weak. Thus, an image of a developer does not efficiently stick to the printing paper P. As a result, transfer quiver occurs. When the transfer voltage is high, the developer is inversely charged. Thus, the image of the developer does not stick to the printing paper P or the electrostatic attraction generated from the second transfer roller 36 is strong. As a result, the image is transferred before the printing paper P approaches the second transfer roller 36. Also, when the transfer voltage is very high, electric discharge may occur. Thus, an appropriate transfer voltage must be applied during a transfer to avoid a poor transfer.
The appropriate transfer voltage varies depending on a resistance (hereinafter referred to as a transfer part resistance) among the transfer backup roller 34, the ITB 30, and the second transfer roller 36. Thus, a method has recently been adopted of measuring a transfer part resistance prior to a transfer, determining a transfer voltage appropriate for the transfer part resistance, and applying the transfer voltage during the transfer.
In a case where a printing paper P does not pass through the second transfer roller 36 at a transfer timing due to a difference between a size of an image and a size of the printing paper P or a jam of the printing paper caused by bad feeding of the printing paper P the image formed on the ITB 30 is transferred to the second transfer roller 36. Thus, the second transfer roller 36 is contaminated by the developer. The resistance of the second transfer roller 36 is increased by the contaminated developer. As a result, a bad transfer occurs during a transfer operation, and a contaminated image stains a rear side of the printing paper P. Therefore, the contaminated developer must be removed from the second transfer roller 36 so as to obtain a uniform, high-quality image.
In a known method of removing the contaminated developer from the second transfer roller 36, an inverse transfer bias, that is, a cleaning voltage, is applied to the second transfer roller 36 in a transfer section so as to transfer the contaminated developer to the ITB 30. The developer inversely transferred to the ITB 30 is cleaned by a cleaning blade 40. However, according to this method, a constant cleaning voltage is used during the inverse transfer regardless of a contamination degree of the second transfer roller 36. Thus, the second transfer roller 36 is not efficiently cleaned.
Also, the contaminated developer sticking on the second transfer roller 36 is squeezed by a pressure applied to the second transfer roller 36 during the rotation of the second transfer roller 36. Thus, the amount of carrier of the developer is reduced, and a density of the toner is increased. However, in a current transfer method, if a density of toner of a developer exceeds 35%, transfer efficiency is sharply reduced. Thus, a transfer and an inverse transfer are not properly achieved. As shown in FIG. 2, if the second transfer roller 36 rotates once or more while contaminated with developer, the density of the toner increases above 35%. Thus, it is very difficult to inversely transfer the contaminated developer on the second transfer roller 36 to the ITB 30 by applying an inverse transfer bias.
Accordingly, there is a need for an improved apparatus and method for cleaning developer from a contaminated transfer roller.